A bone mill is a medical device that, as its name implies, reduces the size of section of bone. The milled bone, which is often chip sized, is used in a medical procedure as a filler adjacent other sections of bone. For example, in a spinal fusion procedure, it is a known practice to place a compound formed out of milled bone around the rods used to hold adjacent vertebra in alignment. This compound serves as a lattice upon which the tissues forming the vertebra grow so as to form a bone-link between these vertebras. This link minimizes the load imposed on the rods. Milled bone is similarly used as filler and/or growth formation lattice in orthopedic surgical procedures and other procedures such as maxillofacial procedures.
Milled bone is used as a filler/growth formation lattice in these procedures because the material, the proteins from which it is formed can serve as make-up material from which the blast cells of the adjacent living bone cells can form new bone. Accordingly, in a surgical procedure in which it is desirable to foster the growth of new bone, milled bone, to which supplemental material is sometimes added, is employed as filler in the spaces in which bone growth is desired.
Milled bone is formed by taking a large mass of bone, a mass that may have a volume of 8 cm3 or more, and reducing it into chips. A chip typically occupies of volume of 0.008 cm3 or smaller.
The bone mill is the device used to mill, morsellize, the large mass of bone to chip size. A typical bone mill includes a housing. A blade assembly or mill head is rotatably mounted to the housing. This blade assembly/mill head is formed with cutting surfaces able to shave or break up the bone pressed against it. There is also some type of device for driving the blade assembly/mill head. If the bone mill is manually operated, the drive device is often a handle capable of rotating the blade assembly/mill. A powered bone mill includes a motor that performs this function.
A bone mill, it should also be understood, is typically designed to be used in the operating room in which the procedure is being performed for which the bone chips are required. This is because, in many situations, the bone used to form the bone chips comes from another portion of the patient's body. The use of the patient's own tissue reduces the likelihood of its rejection by the body. Therefore, an initial part of procedure in which the bone compound is used often involves harvesting a small piece of bone from another portion of the patient's body. This bone is milled into the compound-forming bone chips. This bone harvested from the patient is referred to as autograft bone. Bone from a source other than the patient is referred to as allograft bone.
Ideally, the compound-forming bone chips should undergo as minimal as possible surface oxidation prior to implantation in the body. This surface oxidation results reduces the extent to which the material forming the bone chips can serve as lattice or feedstock that fosters the growth of new bone. Thus, even when allograft bone is milled to form bone chips, this milling process is performed as close as possible to the time at which the chip compound is needed.
Known bone mills work reasonably well. Nevertheless, there are some limitations associated with these devices. Some bone mills, for example are provided with reusable blades/mill heads. One disadvantage of this type of assembly is that after each use, time must be spent to disassembly its components for cleaning and then reassemble them for later use. A further disadvantage of this type of device is that a blade/mill head typically has a number of closely spaced apart surfaces some of which have sharp edges. After each use of the device, care must be taken to carefully sterilize the blade. This process can take an appreciable amount of time in order to ensure both that the blade is properly sterilized and the person perform this task does not cut his/her fingers on the sharp edges.
Furthermore, over time, the blades of a reusable mill invariably dull. This requires one to either by a new blade set or resharpen the existing blades.
To avoid the difficulties associated with sterilizing a bone mill blade, bone mills with use once, replaceable, mill units are available. This type of device includes a base to which a mill unit is removably attached. The mill unit includes a body in which a blade is rotatably mounted. Often the base includes a motor for driving the blade. This type of device is designed so that, after a single use, the mill unit is discarded. An advantage of this type of device is that medical personnel do not have to concern themselves with sterilizing, the blade, a sharp metal object.
Some of these systems are designed so that once the bone is morsellized, the medical personnel have to use either instruments or their fingers to remove the bone from around the blade. Having to perform this step adds to the overall time it takes to provide the bone. Some known disposable bone mills are designed so that in order to access the bone chips, the medical personnel have to remove the blade. In this step of the procedure, personnel have to concern themselves with not getting cut by a sharp metal object.
Further, many known bone mills are constructed so that the blade may repeatedly strike against the same surface of the bone or bone chip. The frictional heat generated by such activity can damage the material forming the bone. This damage can adversely affect the ability of the bone chips to function as growth material for the new bone.
Still another disadvantage of some known bone mills is that the frequency with which the bone milled is reduced in size in the milling process varies within a single milling operation. Some bone may be subjected to only nominal milling. The chips formed as a consequence of this milling may be tool large to be used in the subsequent procedure. Still other bone, in the same milling operation, may, as a result of repetitive milling, be milled to down to very small, almost dust sized particles. The small size of these chips makes their collection for use in the procedure difficult.
Moreover, a sizeable fraction of the bone milled by some bone mills is often not easily accessible for use. This is especially the situation when autograft bone is used to form the bone chips. This particular bone tends to be moist. Consequently, the bone chips have been known to adhere to surfaces of the bone mill, including the actual blade. To ensure that enough bone chips formed from freshly harvested bone is available for use, medical personnel may sometimes feel obligated to form an excess amount of chips knowing that some will not be available for use. Alternatively, these personnel may, once the milling process is completed, have to carefully remove the adhered bone from the surfaces of the mill including the surfaces around the sharp cutting edges of the blade.